1. Field of the Invention
The present invention is related to a calender for a web, e.g., a paper web. The calender includes a stack of rolls having axes that are located in a horizontal plane.
2. Discussion of Background Information
Conventional type calenders, which include a stack of rolls having horizontal axes that are located one above the other in a vertical plane, are used for treating paper, paperboard, plastic foils, and similar material. The nips are either formed by adjacent hard rolls or, particularly in paper refining, are formed by an adjacent hard roll and soft roll. The line load in the nips is provided by hydraulic force transmitters that engage an end roll of the roll stack and by the weight of the rolls. Thus, the line load, and, therefore, the compressive strain, increases from the top to the bottom of the roll stack. In this regard, the variation range of the forces that can be exerted by the force transmitters is relatively small. Further, a limitation of force toward the top of the roll stack must be produced to avoid too intensely stressing the web material in the bottom nip, and to ensure that the web material experiences no further deformation in the top nip.
A calender similar in general to the above described calender is disclosed in WO 95/14813, in which the end rolls are formed with deflection adjustment rolls and the middle rolls are supported on levers engaged by devices for exerting a force. The rolls are designed so that natural deflection lines produced by the weight of the rolls is essentially the same. Further, the nip load generated by the masses of the middle rolls and the associated auxiliary devices is essentially completely canceled out by the devices for exerting force. Theoretically, the entire influence of gravity can be eliminated in this manner, however, in practical terms, extreme difficulties are caused in fulfilling the above-mentioned conditions. In particular, if not all rolls have a same deflection due to their weight, then during operation, higher pressure loads occur on either the edges or in the middle of the roll.
Another calender similar to the above-described calender is disclosed in DE 26 03 484 A1 or DE 26 03 485 A1, substantially avoids the above-noted problems associated with the roll weight. In this calender, the horizontal axes of the rolls are located next to one another so that the weight influence of the rolls on the line load is completely eliminated. The rolls are secured in bearing blocks that can be slid along a guide. One end roll is fixed in place and the other end roll is loaded in the direction of the one end roll by force transmitters in the form of piston/cylinder units. With this calender, the same line load prevails in all of the nips.